The Global Positioning System (GPS) is a global navigation satellite system (GNSS) developed by the United States Department of Defense and is often used for commercial navigation purposes. GPS receivers determine their current location, the time, and their velocity based on radio frequency (RF) transmission from a constellation of satellites. GPS is also a required key synchronization resource of cellular networks, such as for the code division multiple access (CDMA) air interface protocol used by many wireless carriers in a multitude of countries.
GPS receivers can be grouped into two categories. The first group uses the traditional heterodyne receiver architecture, where the RF signal is converted down (“downconverted”) to an intermediate frequency (IF) through mixing with a reference frequency (e.g., one at or near the nominal carrier frequency) before the analog to digital (A/D) conversion takes place. The second group digitally samples (digitizes) the RF signal directly, often at sample frequencies that have the same order of magnitude as the GPS carrier frequency. Both groups require a phase locked loop (PLL) and a locked oscillator (LO) that consume power. However, power might be limited in many applications, e.g., in integrated circuits and chips aboard satellites and other mobile devices.
In sub-sampling architectures, an RF signal is digitized at a frequency close to the much lower frequency of the information content changes rather than at the carrier frequency. Such architectures reduce power consumption of the above approaches because the PLL and LO can be omitted. However, as is well known, sub-sampling architectures have terrible noise figures (˜30 decibels, dB). All noise throughout the receive band is aliased into the sub-sampled frequency band. This makes such architectures unsuitable for GNSS applications, in which a noise figure of less that 4 to 5 dB is typically desired.